1. Field of the Invention
This invention relates to an ink supply roll for a stencil printer.
2. Description of the Related Art
There have been known an outer press type stencil printer and an inner press type stencil printer. The outer press type stencil printer comprises a printing drum around which a stencil is wound, a squeegee roll which supplies ink to the inner peripheral surface of the printing drum, and an outer press roll which presses a printing paper against the outer peripheral surface of the printing drum with the stencil intervening therebetween.
The inner press type comprises a cylindrical printing drum having a flexible porous peripheral wall around which a stencil is wound, an ink supply section provided inside the printing drum, a back press roll provided at a predetermined distance from the printing drum to extend in parallel to the printing drum, and an inner press roller which is provided inside the printing drum and is adapted to outwardly deform the flexible peripheral wall of the printing drum to press the peripheral wall and the stencil thereon against the back press roll. In the inner press type stencil printer, a printing paper is fed between the stencil on the deformed peripheral wall of the printing drum and the back press roller when print is made.
In the outer press type stencil printer, ink is supplied to the inner peripheral surface of the printing drum by way of the squeegee roll and in the inner press type stencil printer, ink is supplied to the inner peripheral surface of the printing drum by way of the inner press roll. In this specification, xe2x80x9cink supply rollxe2x80x9d means various rolls for supplying ink to the inner peripheral surface of the printing drum including the squeegee roll in the case of the outer press type stencil printer and the inner press roll in the case of the inner press type stencil printer.
In a stencil printer, W/O emulsion ink or oil ink is generally used. Since including pigment, solvent, resin and/or activator, such stencil printer ink is low in both polarity and surface tension. Accordingly, the surface layer of the ink supply roll conventionally has been formed of metal or rubber. For ink which is especially low in polarity, a material which is good in wetting, e.g., metal high in surface polarity or rubber, is selected. When rubber is selected, the rubber should be low in swelling.
Various stencil printing inks have been developed according to the needs and progress of printing technique. For example, ultraviolet curing ink has been attracting attention in that since it is cured upon exposure to ultraviolet rays and instantaneously dries, it can contribute to rationalization of the printing process, is free from air pollution by solvent or the like, and can produce film which is strong and high in wear resistance. However, the ultraviolet curing ink is produced by forming a vehicle by photosensitive oligomer and monomer and adding a photo-initiator to the vehicle and is generally high in viscosity.
Use of the ultraviolet curing ink in the conventional stencil printer gives rise to the following problems due to difference in properties from the oil ink and the like which have been generally used in the stencil printer.
First, since the ultraviolet curing ink is apt to be cured when subjected to large frictional force, there is fear that the ink is cured when it is kept sliding for a long time between metal components in the printing drum. Second, since the ultraviolet curing ink is higher in polarity than the conventional stencil printer ink, the ink can soak into the ink supply roll, when the ink supply roll is formed of rubber which has been conventionally used, and swell the ink supply roll to deteriorate the durability of the ink supply roll and to cause mechanical trouble in the components in contact with the ink supply roll. Third, if the ink supply roll is formed of a rubber material low in polarity in order to prevent swelling of the ink supply roll, wetting of the ink to the ink supply roll becomes poor, which can result in blur in the printed image. This tendency is enhanced as the temperature increases since the viscosity of the ultraviolet curing ink becomes lower at an elevated temperature.
In view of the foregoing observations and description, the primary object of the present invention is to provide an ink supply roll for a stencil printer which can prevent curing of the ultraviolet curing ink, is less apt to be swelled by the ultraviolet curing ink, is good in wetting to the ultraviolet curing ink, and is high in durability even when it is exposed to the ultraviolet curing ink.
In accordance with the present invention, there is provided an ink supply roll which is disposed inside a printing drum of a stencil printer and supplies ink to the inner peripheral surface of the printing drum, characterized in that at least its surface layer is formed of a nonmetal material which is not larger than 100xc2x0 in contact angle and not larger than xc2x15% in swelling ratio to an ultraviolet-curing monomer.
The ink supply roll need not be entirely formed of such a material but has only to be formed of such a material at its surface layer. For example, the ink supply roll may be formed by wrapping a band of such a material around a metal roll. In this case, it is preferred that the band be of a substantial thickness taking into account the durability of the ink supply roll. It is needless to say the ink supply roll may be entirely formed of such a material. In this case, the durability of the ink supply roll can be further enhanced.
The term xe2x80x9ccontact anglexe2x80x9d is generally defined as follows. That is, as shown in FIG. 1, when there exists liquid on the surface of a solid in air, out of the angles between the surface of the solid and a tangent which touches the liquid at the point of contact P where the solid phase, the liquid phase and the air phase are in contact with each other, one including the liquid phase (indicated at xcex8 in FIG. 1) is defined to be the xe2x80x9ccontact anglexe2x80x9d. In this specification, the xe2x80x9ccontact anglexe2x80x9d means the contact angle when distilled water at 23xc2x0 C. is used as the liquid. In accordance with the present invention, the contact angle of the skin material of the ink supply roll should be not larger than 100xc2x0 and preferably not larger than 98xc2x0.
As the material which is not larger than 100xc2x0 in contact angle, chloroprene rubber, EPDM rubber (copolymer of ethylene, propylene and diene), fluororubber, NBR (70xc2x0) (acrylonitrile-butadiene rubber) and the like are preferred.
The swelling ratio is generally defined as a change in volume or mass when rubber or the like absorbs gas or liquid. In this specification, the xe2x80x9cswelling ratioxe2x80x9d means a change in mass of a material when the material is dipped in ultraviolet curing monomer at 50xc2x0C. for one week. That is, that the material is not larger than xc2x15% in swelling ratio means that the mass of the material after the dipping is in the range of 0.95 to 1.05 when the mass of the material before the dipping is taken as 1. In accordance with the present invention, the swelling ratio of the skin material for the ink supply roll should be not larger than xc2x15% and preferably not larger than xc2x13%. The materials which is not larger than xc2x15% in swelling ratio include chloroprene rubber, EPDM rubber, fluororubber, natural rubber and the like.
Further, the skin material for the ink supply roll of the present invention should be a nonmetal material which is solid at normal temperatures.
In accordance with the present invention, by using a nonmetal material which is not larger than 100xc2x0 in contact angle and not larger than xc2x15% in swelling ratio to an ultraviolet-curing monomer as the skin material of the ink supply roll, curing of ink due to sliding friction can be suppressed, wetting of ink to the ink supply roll is improved so that blur of image is suppressed, and swelling of the ink supply roll by the ultraviolet curing ink can be suppressed, whereby durability of the ink supply roll can be ensured.